The present invention is directed to a low threshold level radiation detector in which reflective material is directly applied to a radiation detector surface without a separate window cover to allow detection of low levels of radiation.
Radiation detection devices or radiation detectors typically use a window cover such as an aluminum window or an aluminized MYLAR® polyester film window (MYLAR® is a biaxially oriented, thermoplastic film made from ethylene glycol and dimethyl terephthalate (DMT)) to cover the radiation detector components such as Photomultiplier Tubes (PMTs), Geiger-Mueller Tubes (GMTs), or Solid State Detectors (SSDs). An exemplary PMT is shown in FIG. 1 and an exemplary SSD is shown in FIG. 3.
A PMT is a sensitive photocell used to convert light signals of a few hundred photons into a usable current pulse without significantly increasing noise. A PMT may include a photocathode coupled to an electron multiplier structure contained within a glass envelope. The photocathode may include a photosensitive layer that converts as many of the incident light photons as possible into low-energy electrons. The number of photoelectrons produced will be comparable to the number of incident light photons and thus, the charge on the photoelectrons will be too small to provide a detectable electrical signal. The electron multiplier section may include an arrangement of dynodes that serves both as an efficient collection geometry for the photoelectrons and a nearly ideal amplifier that greatly increases the number of electrons. After amplification, a typical scintillation pulse will give rise to 107–1010 electrons, sufficient to generate a charge signal that can be collected at the anode. PMTs in general, perform a highly linear charge amplification producing an output pulse that, over a wide range, is proportional to the number of original photoelectrons. Much of the timing information of the original light pulse is also retained. When illuminated by a very short duration light pulse, most tubes will produce an electron pulse with a time width of a few nanoseconds after a delay of only 20–50 ns. For exemplary purposes only, one PMT is the R647-01 that is made and/or sold by Hamamatsu of Japan.
An SSD generally consists of a piece of silicon, germanium, cadmium zinc telluride (CdZnTe), or other material that emits an electrical signal in response to ionizing radiation. SSDs may be, for example, an X-ray detector that uses a scintillation crystal coupled to a solid-state photodiode to convert the incident X-ray intensity to an electric current. Solid state detectors have very high X-ray stopping efficiency (compared to the xenon detector) and are compact (compared to detectors using a photomultiplier tube). For exemplary purposes only, one SSD is the PIN10D1 that is made and/or sold by UDT Sensors, Inc. of Hawthorne, Calif.
As mentioned, current radiation detectors use window covers to cover the radiation detector components. The thickness of the window cover material will attenuate low energy radiation. The amount of attenuation depends on the strength and type of radiation and the thickness and type of material used for the window. Typical aluminum windows are approximately 50–75 microns thick. Typical aluminized MYLAR® windows are approximately 50–75 microns thick with a 0.08 micron thick aluminum material added for light reflection. The window cover serves three purposes. First, the window cover blocks ambient light from entering the radiation detector. Ambient light could cause errors in the measured radiation. The external light may also cause temporary/permanent damage to sensitive components. Second, the window cover focuses scintillated light into the measuring device. Scintillators will emit a photon in any direction. Photons that are emitted out of the measurement device will be redirected to make a correct measurement. This is especially important in the PMTs. Third, the window cover protects sensitive components from damaging agents such as gasses and solids in the measured medium.
Due to the window cover's thickness, however, the window cover also attenuates a portion of the radiation being measured (e.g. low energy radiation) in that the energy cannot penetrate the window cover. This attenuation can reduce the amount of measured radiation that is detected and/or measured. In other words, the attenuation caused by the thickness of the window cover can introduce significant errors in results produced by the radiation detection devices. Accurate detection of radiation is critical and erroneous results can be dangerous.